1. Field of the Invention
The present invention relates to a brush holder device for a dynamoelectric machine.
2. Description of Related Art
In one previously proposed brush holder device for a dynamoelectric machine (e.g., a motor, a dynamo), each brush is slidably received in a corresponding brush holder, which is secured to a base plate, in such a manner that the brush is urged against a commutator of an armature by a spring to slidably engages the commutator.
Japanese Unexamined Utility Model Publication No. 5-4750 discloses one such brush holder device for a motor. In this brush holder device, a metal plate material is bent and thus has a square horseshoe shaped cross section, so that the brush holder is formed. A lower opening of the brush holder is oriented toward the base plate (a brush holder stay). Furthermore, securing claws of the brush holder are inserted through corresponding securing holes of the base plate and are bent against the base plate to secure the brush holder to the base plate by staking.
In a case where the base plate is formed from a dielectric resin material to achieve a light weight and high moldability of the base plate, the brushes are in direct contact with a top surface of the base plate. When the motor is rotated, the brushes are heated to a relatively high temperature. Due to the direct contact of the brushes with the base plate, the heated brushes may cause melting and thermal deformation of the top surface of the base plate. Such deformation of the top surface of the base plate, in turn, may prevent smooth movement of one or more brushes in the corresponding brush holder, causing generation of a spark or noise. To address such a disadvantage, the base plate can be made from a heat resistant resin material. However, the heat resistant resin material is generally expensive, resulting in increased manufacturing costs.
U.S. Pat. No. 6,229,242 discloses a brush holder device, which addresses the above disadvantage. In this brush holder device, a metal base plate is secured to a lower opening of a square horseshoe shaped brush holder (a cover plate), i.e., is secured to the top surface of the base plate (the brush holder plate) to prevent direct contact between the brush and the base plate. With this arrangement, even when the base plate is made from the inexpensive resin material, which has the low heat resistivity, the melting and thermal deformation of the base plate, which could be caused by the high temperature of the heated brushes upon rotation of the motor, can be limited since the brushes do not directly contact the base plate.
However, in the case of the brush holder device disclosed in U.S. Pat. No. 6,229,242, the metal base plate is additionally required for each brush. Thus, the number of components and the number of assembling steps are disadvantageously increased.
Furthermore, in the above case where each brush holder is formed by bending the metal plate, when the brush, which is received in the brush holder, is in sliding engagement with the rotating commutator, the brush is vibrated in the rotating direction of the commutator. The vibration of the brush tends to increase a space between two lateral walls of the brush holder. Thus, it is also demanded to limit the increase in the space between the lateral walls of the brush holder in a easy and effective way.
Furthermore, in one previously proposed brush holder device of the above kind, a circuit breaker is provided to prevent seizing (burnout) of the motor during rotation of the motor.
In the description of prior art of Japanese Unexamined Patent Publication No. 9-107651, one such prior art brush holder device, which has the circuit breaker secured relative to a base plate of the brush holder device, is recited. More specifically, the circuit breaker is held by a breaker holder, which is formed separately from the base plate and is installed to the base plate. However, in this case, the number of components and the number of manufacturing steps of the brush holder device are disadvantageously increased, resulting in an increase in the manufacturing costs of the brush holder device.
To address the above problem, Japanese Unexamined Patent Publication No. 9-107651 proposes one brush holder device. In this brush holder device, one of two terminals of the circuit breaker is connected to a pigtail, which extends from a brush, and the other one of the terminals is securely connected to one end of a preexisting conductive terminal member (lead plate). The other end of the terminal member is secured to a motor housing together with the base plate by a conductive screw, so that the other end of the terminal member is grounded to the motor housing, which is, in turn, grounded to a vehicle body. In this way, the terminal member is used to electrically connect the circuit breaker to the ground and is also used to secure the circuit breaker. Thus, the breaker holder is no longer required in the brush holder device. As a result, the number of manufacturing steps is reduced to reduce manufacturing costs of the brush holder device.
The above circuit breaker includes a cover, a case and a switch member. The one of the terminals extends from the cover. The other one of the terminals is provided in the case. The switch member is received in the case and is positioned between the case and the cover. The switch member stops flow of drive current in the motor when the drive current becomes equal to or greater than a predetermined value. In the circuit breaker, an opening of the case is closed by the cover through an insulating paper to cover the switch member received in the case. Thus, the case and the cover are electrically insulated from one another.
However, the two terminals project in opposite directions. Thus, at the time of connecting a lead wire or a terminal of the base plate to the terminals of the circuit breaker, a moving distance of a connecting machine for connecting them is disadvantageously increased. Particularly, in a situation where the case extends from one axial side of the base plate to the other axial side of the base plate such that the two terminals of the circuit breaker are disposed on the axial sides, respectively, of the base plate, the connecting operation of the connecting machine needs to be performed on each of the axial sides. Furthermore, lately, many brush holder devices have noise limiting elements, and the motors are downsized. Thus, spaces for accommodating the circuit breaker and the terminal member are limited. As a result, it is demanded to arrange the case to extend from the one axial side to the other axial side of the base plate to reduce a space required to accommodate the case. Because of the above reasons, it has been demanded to arrange the cover of the circuit breaker and the terminal member close to one another while maintaining a sufficient space therebetween and electrical insulation therebetween. In such a case, when the terminal member is connected to the other one of the terminals of the circuit breaker, the terminal member could be unintentionally placed to contact the cover due to, for example, vibrations of the motor. When the terminal member contacts the cover, the terminal member and the cover are electrically connected to one another, i.e., are shortcircuited without passing through the switch member, disenabling the function of the circuit breaker. Particularly, as described above, when the case of the circuit breaker extends from the one axial side to the other axial side of the base plate, the terminals of the circuit breaker can extend from the same side of the circuit breaker to improve the connecting operation. However, since the terminals of the circuit breaker are held close to one another, and the terminals of the base plate to be connected with the terminals of the circuit breaker overlap with the circuit breaker, it is difficult to achieve reliable insulation between the cover and the terminal member.